The present invention relates to the field of optical fiber transmission, and more particularly to the field of transmission with wavelength division multiplexing (WDM).
In optical fibers, the refractive index profile is generally described in terms of the appearance of the graph of refractive index as a function of the associated radius. Conventionally, distance r from the center of the fiber is plotted along the abscissa and refractive index is plotted up the ordinate in terms of its absolute difference or its percentage different relative to the index of the fiber cladding. Thus, the profile can be said to be xe2x80x9csteppedxe2x80x9d, or xe2x80x9ctrapezium-shapedxe2x80x9d, or xe2x80x9ctriangularxe2x80x9d, for curves representative of index variation as a function of radius having respectively a stepped shape, a trapezium shape, or a triangular shape. These curves are generally representative of an ideal or reference profile for the fiber, fiber manufacturing constraints possibly leading to a profile that is perceptibly different.
In new high data rate transmission networks using wavelength division multiplexing, it is advantageous to manage chromatic dispersion, particularly at data rates greater than or equal to 10 Gbit/s. The idea is to obtain cummulative chromatic dispersion for all of the wavelengths in the multiplex that amounts substantially to zero over the link, so as to limit the extent to which pulses are broadened. An acceptable value for the cumulative dispersion is a few hundred picoseconds per nanometer (ps/nm). It would also be advantageous to avoid having zero values for chromatic dispersion in the vicinity of the wavelength used in the system since non-linear effects are greater at such values. It is thus preferable to use so-called non-zero dispersion shifted fiber (NZ-DSF) having a zero chromatic dispersion wavelength xcex0 lying outside the range occupied by the channels of the multiplex so as to avoid problems due to four-wave mixing. In addition, the use of such fibers for WDM systems leads to seeking a chromatic dispersion slope that is shallow, so as to conserve similar propagation characteristics for different channels. From this point of view, the reference value for chromatic dispersion slope is 0.075 ps/nm2xc2x7km.
In xe2x80x9cDispersion flattening in a W fiberxe2x80x9d, published in Applied Optics, Vol. 33, No. 6, pp. 1011-1014 (1994), Richard Lundin describes a method enabling flat dispersion to be obtained in monomode fibers having two cladding layers. The method is applied to fibers having an index profile with a rectangular core and a ring of index lower than that of the cladding, and it makes it possible to obtain a mean value for dispersion of 0.9 ps/km.nm over the wavelength range of 1250 nm to 1600 nm, with a cutoff wavelength of 1250 nm. That document merely states that the fact of maintaining chromatic dispersion at a low value makes it possible to restrict pulse broadening.
In xe2x80x9cTriple-clad single mode fibers for dispersion flatteningxe2x80x9d, published in Optical Engineering, Vol. 33, No. 12, pp. 3999-4005 (1994), Y. Li and C. D. Hussey describe the advantage of flat dispersion fibers for WDM transmission systems, with chromatic dispersion reducing to zero at 300 nm and at 1550 nm. That document proposes optimizing triple-clad fibers to obtain flat dispersion, with chromatic dispersion zeros at 1300 nm and at 1550 nm, low intrinsic losses, and good sensitivity to bending. The cutoff wavelength for the second mode is 1250 nm. The fibers that are obtained present a core of index greater than that of the outer cladding, and second cladding of index greater than that of the outer cladding. The losses induced by winding the fiber at a radius of 5 cm are less than 2.4 dB/km, and dispersion is less than 3 ps/nmxc2x7km over the 1300 nm to 1550 nm wavelength range.
EP-A-0 368 014 describes another fiber having the same type of profile. Its chromatic dispersion in the range 1280 nm to 1560 nm is less than or equal to 3.5 ps/nmxc2x7km, and its sensitivity to bending is less than 10xe2x88x926 dB/m for a radius of 5 cm.
EP-A-0 131 634 describes yet another fiber having the same type of profile. That fiber has chromatic dispersion with three dispersion zeros, and opposite maximum and minimum values relating to dispersion between the zeros.
In xe2x80x9cWorldwide status of dispersion modified single mode fibersxe2x80x9d, published in Philips J. Res. 42, 435-450 (1987), P. K. Bachmann proposes a summary of flat dispersion fibers and of dispersion-shifted fibers (DSF). The author concludes that it is necessary to reduce the losses induced by bending the fibers.
The invention proposes a fiber which has characteristics that are appropriate for use in wavelength division multiplexed transmission systems, without inducing large amounts of attenuation, and limiting non-linear effects. The invention applies to all kinds of pulse WDM transmission systems, in particular to RZ or NRZ pulse systems.
More precisely, the invention proposes a dispersion shifted monomode optical fiber which, in a wavelength range of 1400 nm to 1650 nm, presents both a chromatic dispersion maximum and a chromatic dispersion slope of absolute value less than 0.05 ps/nm2xc2x7km.
In an embodiment, the maximum is the sole maximum in the wavelength range 1400 nm to 1650 nm.
Preferably, the fiber presents chromatic dispersion having a positive value at said maximum.
In which case, the maximum is advantageously reached at a wavelength lying in the range 1530 nm to 1580 nm.
The fiber may present a chromatic dispersion zero for a wavelength of less than 1450 nm.
Advantageously, it also presents a chromatic dispersion zero for a wavelength greater than 1600 nm.
In another embodiment, the fiber presents chromatic dispersion having a value that is negative at said maximum.
In which case, the maximum is preferably reached at a wavelength lying in the range 1480 nm to 1520 nm.
In addition, in the wavelength range 1530 nm to 1580 nm, the fiber may present chromatic dispersion having a slope that is positive.
In an embodiment, in the wavelength range 1530 nm to 1580 nm, the fiber presents chromatic dispersion having a slope that is negative.
In another embodiment, in the wavelength range 1530 nm to 1580 nm, the fiber presents chromatic dispersion having a slope of absolute value less than 0.03 ps/nm2xc2x7km.
Advantageously, at a wavelength of 1300 nm, the fiber presents chromatic dispersion having an absolute value of less than 7 ps/nmxc2x7km.
In an embodiment, the fiber presents an index profile having a fiber core and cladding, the core comprising a central portion of index n1 greater than the index ns of the cladding of the fiber, an annular portion around said central portion and having an index n2 less than the index of the cladding, and a ring around said annular portion of index n3 greater than that of the cladding.
In addition, the fiber further presents another annular portion around said ring, and having an index n4 less than that of the cladding.
Advantageously, the difference between the indices of the central portion and the annular portion lies in the range 13xc3x9710xe2x88x923 to 17xc3x9710xe2x88x923.
Preferably, the difference xcex94n2 between the index n2 of the annular portion and the index ns of the cladding lies in the range xe2x88x928xc3x9710xe2x88x923 to xe2x88x926xc3x9710xe2x88x923.
In an embodiment, the difference xcex94n3 between the index n3 of the ring and the index ns of the cladding lies in the range 3xc3x9710xe2x88x923 and 6xc3x9710xe2x88x923.
Advantageously, the radius a1 of the central portion is less than or equal to 3 xcexcm.
Preferably, the ratio (a2xe2x88x92a1)/a1 between the thickness of the annular portion and the radius of the central portion lies in the range 0.8 to 1.2.
In an embodiment, the ratio (a3xe2x88x92a2)/a1 between the thickness of the ring and the radius of the central portion lies in the range 0.3 to 0.7.
Provision can also be made for the difference xcex94n4 between the index n4 of the other annular portion and the index ns of the cladding to lie in the range xe2x88x920.5xc3x9710xe2x88x923 and xe2x88x920.1xc3x9710xe2x88x923. 
The invention also provides the use of such a fiber for wavelength division multiplexed transmission systems.